1. Field of the Invention
This invention relates to the field of electroplating. More specifically, the present invention relates to an apparatus and a method for improving the process of electroplating of controlled collapse chip connection (C4) microbumps, of tape automated bonding (TAB) microbumps, and of integrated circuit interconnects.
2. Related Art
Typically, integrated circuits are assembled by using the standard "wirebond" integrated circuit method which will be explained in conjunction with FIGS. 1a-1d. FIG. 1a shows a wafer 10 with dice 12 laid out in horizontal and vertical rows. A die can be singled out, as shown in FIG. 1b, and bonded into a Pin Grid Array package (PGA) 14 shown in FIG. 1c. As one can see from this figure, first the die 12 is positioned face up within a recess 17 made onto the top surface 16 of the PGA 14. Then the die is bonded to the PGA 14 by means of the wirebond pads 18. The wirebond pads 18 are also connected to various functional blocks of the die via conductors (not shown). Typically, up to 600 wirebond pads are positioned around the periphery of the die for making connections with the package. A wirebonder is used to wirebond the die, via the wirebond pads 18 to the wirebond pads 19 positioned onto the top surface 16 of the PGA 14. The wires are looped up and over to the corresponding pads on the package as shown in FIG. 1d. One drawback of this method is that the routing of conductors, from the functional blocks of the die to the peripheral wirebond pads 18, increases signal propagation delays due to the high impedance of the conductors.
A more recent technology called controlled collapse chip connection ("C4") has emerged in the field of integrated circuit assembly. According to this method, the chip is positioned face down in the package, instead of face up. The die is then connected to the package via a plurality of solder bumps which are soldered to both the die and the package. The connections between the package and the die are made straight down into the package, as opposed to the connections in the conventional wirebond technology where the connections were made by routing conductors or wires to the peripheral wirebond pads, and then looping wires up and over to the package. Due to the shorter signal path from the die to the package, this method provides an improvement over the old method with respect to the parasitic impedances of the conductors.
According to one method for depositing solder bumps onto a wafer, a molybdenum mask 30 is overlaid onto the wafer 10 and then aligned to the input/output of each die 12 as shown in FIG. 2. The molybdenum mask has openings 32 at locations corresponding to the projected signal connections on the die. The wafer 10 and the overlaid mask 30 are inserted into a vacuum chamber and exposed to physical vapor deposition. A compound such as lead and tin, for example, is heated until it melts, evaporates inside the vacuum chamber, and deposits through openings 32 onto the wafer, thereby forming bumps. Once the compound is deposited, the molybdenum mask is torn off the wafer. The C4 bumps remain on the wafer at the desired locations corresponding to the respective openings in the mask thereby defining the desired microbump pattern in the molybdenum mask. The problem with this process is that a lot of the material is wasted during the deposition. Almost 90-98% of the material is deposited on the vacuum chamber's walls instead of being deposited onto the wafer.
A more advantageous method is electroplating solder onto the wafer. This method involves initially metalizing the surface of the wafer thereby forming a conductive layer across the wafer. The next step is depositing a photoresistive mask which defines a predetermined bump patterns, upon the surface of the conductive layer. The photoresistive material electrically insulates the metal layer except for the openings through the photoresist where bumps are to be plated. The following step is plating the bumps with an alloy of lead and tin. The bumps build up in a predetermined bump pattern, each bump having a desired final height. The next step is stripping the photoresistive material and also stripping the portions of the conductive layer which do not have bumps plated thereon so that the bumps will not be short-circuited.
The system used for the above-described electroplating process is configured utilizing a cup which contains the electrolyte and holds the wafer in place during the electroplating process. Electrolytic plating requires, among other things, an electrolyte which contains lead and tin in an ionic suspension, for example. Other metals however, depending on the specific application, can be used in the ionic suspension. Furthermore, electrolytic plating requires electrical contacts onto the wafer such that a negative charge will be distributed onto the conductive metal layer of the wafer. The negative charge flowing onto the wafer combines with positive ions from the electrolytic solution, through a reduction process, thereby causing the lead to be deposited onto the wafer in the form of microbumps positioned onto the wafer according to a predetermined microbump pattern. Typically, there are two types of currents which flow onto the wafer-cathode current and anode current. The cathode current is provided by the cathode along the surface of the wafer. The anode current, provided by the anode assembly, flows in a direction substantially transversal to the plane of the wafer towards the wafer to be plated.
The above-described method, however, suffers from several disadvantages. One such disadvantage is non-uniform cathode current flowing onto the cathode surface of the wafer. The non-uniform cathode current causes non-uniformity in the anode current which in turn causes the microbumps formed onto the wafer to display non-uniformity across the wafer. The non-uniform cathode current flowing onto the wafer is caused, mostly, by the fact that prior art cathode contacts were connected to the wafer at only a discrete number of points located at the periphery of the wafer.
Non-uniform bumps can cause several problems. For example, if the chip has bumps that are very small and bumps that are very large, the die will be tilted upon its assembly onto the package. The lack of uniformity in the bump pattern also causes open circuits due to the fact that not all microbumps are connected to the pads of the package. Yet another problem occurs where bumps are formed so close to each other causing a short circuit. This type of problem is also known as bridging bumps.
Furthermore, non-uniform distribution of negative charge onto the target surface can affect the deposition of metallic particles in cases other than microbumps electroplating. For example, integrated circuit interconnects are also plated onto silicon wafers. Non-uniformity of the cathode current can cause non-uniform thickness of the plated interconnect. This, in turn, can cause undesirable effects such as electro-migration.
Furthermore, some electroplating methods require that the cathode contact, which is mounted on top of the cup, is easily removable once it is used up. However, removal of the cathode contact can be very difficult since the cathode contact will stick to the rim of the cup holding the cathode contact due to lead tin, or other metal, deposited between the cathode contact and the rim of the cup.
Consequently, a different type of configuration for the cathode contact is desirable such that the anode current flowing across the wafer will be uniformly distributed across the surface of the wafer. Moreover, it is desirable to have a cathode contact which is protected from particle deposition thereon. Furthermore, it is desirable to have a cathode contact which is easily removable once this contact needs to be replaced. Also, it is desirable to have a cathode contact which provides a resilient sealing against the photoresist layer of the wafer--thereby protecting, from deposition during plating, the exposed conductive metal contact located at the edge of the wafer. It is also desirable that the cathode contact be very thin in order to minimize the obstruction of smooth flow of the electrolyte at the periphery of the wafer. Also, it is desirable to have a cathode contact which is easily and precisely positioned onto the electroplating cup relative to a conductive contact of the wafer located at the periphery of the wafer.